"MAP receiver for high-speed numerical transmissions through Rayleigh channels noisy and dispersive in time and frequency"
The present invention relates to a receiver of digital signals propagated in a mobile radio communication channel having multiple paths of the varying time type which give rise to distortions in time and frequency with uncertain characteristics having Rayleigh distribution.
As known, a Rayleigh channel is usually defined as a transmission channel in which a signal subject to fast fading with Doppler effect varies in amplitude, frequency and phase in accordance with a statistical law derived from the .chi..sup.2 probabilistic distribution.
Given the specialization of the subject, such that it is often necessary for reasons of simplicity to use initials or abbreviated expressions, for any clarifications on the present description reference is made to the numerous publications existing on the subject such as for example:
Digital Communications by J. Proakis, PA1 Data Communications Principle by Gitlin, Hayes and Weinstein, and PA1 Introduction to Random Signals and Applied Kalman Filtering by Brown and Hwang.
The most important causes of worsening introduced in the signal which transits in a high-speed numerical transmission channel consist of the linear distortion or intersymbolic interference (overlapping of symbols belonging to adjacent transmission blocks, abbreviated hereinafter ISI), rapid time variations and severe fading introduced by the multiple paths present.
This occurs especially in high frequency radio links where channel variation is due to the variation in time of the height of the ionosphere strata and in the mobile radio communications where it is introduced by the speed of the vehicle.
In such environments non-linear adaptive equalization is obligatory to avoid serious worsening of the digital link due to the so-called multiple paths phenomenon.
There are two classes of adaptive equalizer most commonly used, i.e. the Decision Feedback Equalizer (DFE) and the receiver with Maximum Likelihood Sequence Estimate (MLSE) based on the Viterbi algorithm (briefly VA, an algorithm which permits calculation in reception of the most likely sequence of the symbols transmitted to reduce the number of calculations optimizing decodification of the convolutional codes).
Strategies based on adaptive DFE have been amply studied in recent years and although they can be considered attractive for many applications due to their simple structure it is known that they are largely sub-optimal compared with solutions based on MLSE in environments having rapid time variations because of the so-called catastrophic events caused by error propagation. However, if the time variations are fast enough the MLSE equalizer is unable to supply a sufficiently accurate estimate of the channel response. To obviate this shortcoming there have recently been proposed new types of MLSE equalizers based on the so-called principle of path survival calculated by the decoder at reception (Path Survival Processing, abbreviated PSP, see for example H. Kubo, K. Murakami, T. Fujino "An adaptive Maximum Likelihood Sequence Estimator for fast varying Intersymbol Interference Channels", IEEE Trans.on Comm., vol 43, n. 2/3/5, pages 1872-1880, 1994; R. Raheli, A. Polydoros, C. K. Tzou "Path Survivor Processing: A General Approach to MLSE in Uncertain Environment", IEEE Trans. on Comm., vol 43, n. 2/3/5, pages. 354-364, 1995).
The performance of these equalizers is undoubtedly good as concerns fast time variations but their implementation is very costly due to the high number of channel estimators necessary.